Abstract The intermingling and bi-directional transfer of maternal and fetal cells during gestation creates a fascinating and highly relevant physiological model of immunological tolerance. Although this interaction has primarily been addressed from the perspective of maternal tolerance to foreign paternal antigens expressed by the developing fetus, the fetus is also reciprocally exposed to an equally vast array of genetically foreign non- inherited maternal antigens (NIMA). Thus, potent regulatory pathways are likely in place beginning at the earliest stages of development in individuals to maintain immunological tolerance to genetically foreign NIMA. Our analysis of T cells with specificity to model antigens transformed into surrogate NIMA show compulsory developmental exposure to genetically foreign maternal antigen primes expanded accumulation of immune suppressive regulatory CD4+ T cells (Tregs) with NIMA-specificity. Selectively enriched NIMA-specific Treg accumulation in female offspring is associated with reinforced fetal tolerance during next-generation pregnancies sired by males expressing allo-antigens with overlapping NIMA specificity. Importantly, protection against fetal wastage and expanded NIMA-specific Treg accumulation requires persistent cognate antigen stimulation by maternal cells that establish microchimerism in offspring. Thus, genetically foreign maternal cells vertically transferred and retained in offspring maintain expanded peripheral tolerance to NIMA. Based on these exciting proof-of-concept data establishing the pivotal importance of microchimeric maternal cells in NIMA-specific tolerance and cross-generational reproductive fitness, we are exceptionally well poised to investigate how microchimeric cells naturally transferred between mother and child confer antigen-specific immunological tolerance. Aim 1 will use newly validated cell enrichment techniques combined with multi- parameter flow cytometry to establish the identity and molecular properties of microchimeric maternal cells responsible for NIMA-specific tolerance. Complementary studies will investigate the necessity for microchimeric cell-intrinsic expression of molecules required for T cell antigen presentation and/or co- stimulation/co-inhibition in maintaining NIMA-specific tolerance. To address the durability of NIMA-specific tolerance and extend the relevance for these loss of function studies for microchimeric maternal cells, Aim 2 will investigate physiological contexts where functional immune tolerance to NIMA might be naturally ?erased?. In particular, we will determine how NIMA-specific tolerance can be displaced in female offspring by fetal microchimeric cells seeded during next-generation pregnancies. Lastly, Aim 3 will evaluate whether expanded tolerance is restricted to genetically foreign antigens expressed by microchimeric cells, or more broadly extends to tissue restricted self-antigens responsible for causing autoimmunity by investigating shifts in autoimmune disease susceptibility when the inciting tissue-restricted self-antigen is transformed into a surrogate NIMA through forced constitutive expression by tolerogenic microchimeric maternal cells.